1. Field of the Invention
The present disclosure relates to an image forming apparatus.
2. Description of the Related Art
Conventionally, an electrophotographic image forming apparatus employing what is called contact charging process has been put to practical use, which has advantages such as low ozone, and low electric power. The contact charging process involves application of voltages on a charging roller serving as a charging member which contacts a photosensitive member serving as an image bearing member, thereby charging a surface of the photosensitive member. Furthermore, in recent years, from viewpoint of low costs, and space-saving, there is employed DC charging process using a charging roller to which only direct current (DC) voltage is applied.
In the DC charging process, there is a problem that uniformity of charging is insufficient. In the contact charging process, there is a problem that a surface potential of the photosensitive member rises when a film thickness of the surface of the photosensitive member decreases along with frequent use of the photosensitive member.
In order to solve these problems, there is known an exposure method of overcharging once the surface potential of the photosensitive member to equal to or greater than an electric potential necessary for image formation, after that, irradiating with weakly emitted laser light, a non-image region where image formation on the surface of the photosensitive member is not performed, thereby lowering the potential. This method is called “background exposure” (Refer to Japanese Patent Application Laid-Open No. 8-171260). Further, Japanese Patent Application Laid-Open No. 2002-296853 discusses a control method for forming a surface potential of the photosensitive member to be a targeted potential by calculating a film thickness of the photosensitive member, and controlling a laser power of the laser light. By performing such a control, image density, line width, and gradation can be stably reproduced
On the other hand, in the above-described electrophotographic image forming apparatus, as a process for forming a color image, a color image formation process what is called a tandem type is commonly used. In an image forming apparatus of the tandem type, toner images of respective colors of yellow, magenta, cyan, and black are formed on the photosensitive members. Then, in the image forming apparatus of such a tandem type, process units such as a charging device and a development device each are individually arranged on each of the photosensitive members that form the toner images of the respective colors.
In the image forming apparatus of the tandem type that enables image formation of a plurality of colors as described above, a configuration of sharing as much as possible a power source for each of the charging unit, and development unit is preferred, from viewpoint of reduction of size, and reduction of cost. However, in the image forming apparatus of the DC charging process in which the power source is shared, a predetermined charging potential is constantly formed on all the photosensitive members, and light sensitivity can worsen in some cases resulting from a light fatigue of the photosensitive members. Therefore, improvement to reduce worsening of the light sensitivity due to the light fatigue of the photosensitive members needs to be made. Further, in order to perform background exposure, it is necessary to change the surface of the photosensitive member to a desired potential by a predetermined laser power, and a light source as an exposure unit is easily exhausted, and therefore improvement needs to be made from viewpoint of service life extension